The present invention is in the field of ceramics; more particularly the invention relates to ternary silicon-rare earth nitrides and a process for their preparation.
Lanthanum silicon nitride, LaSi.sub.3 N.sub.5, has been disclosed in Inoue et al., "A Crystallographic Study of a New Compound of Lanthanum Silicon Nitride, LaSi.sub.3 N.sub.5 ", J. of Mat. Sci., Vol. 15, pg. 2915, Chapman & Hall Ltd., (1980). The compound has been indicated to be prepared by a reaction between Si.sub.3 N.sub.4 and La.sub.2 O.sub.3 under a 50 atmosphere nitrogen pressure at 2000.degree. C. for 2 hrs. in a pressure furnace. Inoue et al. presents a crystallographic study of this lanthanum silicon nitride compound. Inoue recognizes that other compounds from the same column of the periodic table have been known. However, these compounds contain oxygen. One such compound disclosed is yttrium silicon oxynitride.
In another paper, "A New Role for Nitrogen and Silicon Nitride and Related Ceramics", J. of Mat. Sci. Letters, Vol. 4, pg. 656, Chapman & Hall Ltd., (1985), Inoue addresses silicon nitride and related ceramics and their structure.
Holcombe et al., "Advanced Ceramic Materials for Hydrogen Fluorine Environments", Am. Ceramic Soc. Bull., Vol. 60, pp. 546-548, (1981), disclosed that 10 lanthanum containing compounds or composites were investigated in a controlled F.sub.2 /H.sub.2 flame. Included in the evaluation was LaN.Si.sub.3 N.sub.4. This article concluded that all the materials studied exhibited maximum use temperature greater than that for nickel. The lanthanum hexaboride composites were found to be the best materials.
Carbothermal reduction syntheses of single metal nitrides such as Si.sub.3 N.sub.4 and AlN are well known. This process has also been used for the synthesis of beta-sialons (Si--Al--O--N compounds) using mixtures of SiO.sub.2, Al.sub.2 O.sub.3 and carbon. However, the use of carbothermal reduction for the preparation of ternary metal nitride compounds which do not contain oxygen (except as an impurity) has not been reported.
Although the chemistry of the Ln--Si--O--N system has been previously studied, the carbothermal reduction of mixtures of rare earth oxides with silicon dioxide has not been reported. Wills et al. (J. Mat. Sci., Vol. 11, (1979) pp. 749-759) have, for example, shown that the reaction of rare earth oxides with silicon nitride leads to the formation of rare earth silicon oxynitrides. Similar compounds have been claimed to form during the densification of silicon nitride using rare earth oxides as sintering aids. Thus, the carbothermal reduction process is unique in allowing the synthesis of ternary metal nitrides rather than the formation of oxynitrides.
Non-oxide compounds such as silicon nitride are generally difficult to densify in the absence of sintering additives. These additives, usually metal oxides, promote sintering by formation of liquid phase with a liquidus temperature below the processing temperature. However, the residual intergranular phase leads to reduction in the high temperature strength and renders the material susceptible to creep damage.